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Was it ever considered to optimise a jet to normally operate with one engine shut off?
As I understand it, the current trend to reduce all commercial aircraft to no more than two turbofans has long-relevant reasons – the more engines, the higher the fuel consumption – and earlier jets mostly had to come with more to ensure reliability.
Now, the naïve solution to design an aircraft with 3 or 4 engines but needing only two has obvious economic flaws and a normally mounted engine introduces so much drag that it's certainly always better to run it than not. However it is not unheard that existing aircraft designs were upgraded with better engines (particularly for economic reasons: stern look at 737MAX). I'm thinking particularly of trijets here: the 727 for instance did have its outboard engines upgraded to higher-bypass ones in its career. I could well imagine that they thought about whether the center engine could be made obsolete altogether – just make the new engines 50% more powerful each, and thus degrade the center one to a mere emergency backup. (And do something aerodynamic about the S-duct so it wouldn't be too detrimental for drag anymore.)
Was any such redesign seriously pursued by one of the major manufacturers?

It seems unlikely to be worth the dead weight of carrying a "spare" engine around. Jet engines don't start in an instant, for one. Another reason is that the APU can power many of the systems that the engines usually power, while being smaller and lighter than a full engine.

I can think of one example: the Hawker Siddeley Trident 3B.
This had a fourth, small engine buried in the tail, which was only used during takeoff. They did this because it was cheaper than redesigning the aircraft for different engines.

Since the Spey 512 was the last of the Spey line, extra thrust would be difficult to obtain. Instead of attempting to replace the three engines with a completely different type, which would have been difficult with one engine buried in the tail, Hawker Siddeley's engineers decided to add a fourth engine in the tail, the tiny Rolls-Royce RB162 turbojet, fed from its own intake behind a pair of movable doors. The engine added 15% more thrust for takeoff, while adding only 5% more weight, and it would only be used when needed. BEA accepted this design as the Trident 3B, and ordered 26.

You've got a bit of extra or something missing there at the end of your quote...

Not anymore, thanks ;)

It was considered to operate a mixed propulsion aircraft, Convair B-36, with all jet engines shut down. This aircraft had both propeller and jet engines. While the jet engines greatly improved takeoff performance and dash speed over the target, they where shut down to conserve fuel in a normal cruising flight.
When the jet engines were shut down, louvers closed off the front of the pods to reduce drag.


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Where can I find registration information of a private aircraft registered in Germany?
Is there any way (website, database etc.) where I can get the registration information of a privately-owned general aviation aircraft registered in Germany?
(Germany has pretty extensive privacy laws, but it would seem that aircraft registrations should be a little bit transparent and verifiable...)

What information do you need to know, and what information do you have on the aircraft (e.g. Mode S address, registration, type)

I have the D-XXXX registration, but wanted to know the certification class, owner, manufacturing year, etc. (The sort of thing that the Transport Canada database provides)

The document is called "Luftfahrzeugrolle" and held by the LBA. They can only give information to a third party if the owner agrees to it and if a fee is paid. Here is the German Wikipedia entry on that topic.

I don't think the information is accessible on a public website.
According to the German wikipedia the Luftfahrtbundesamt (LBA, the German Civil Aviation Authorities) cannot publish the information for data protection law reasons.
On their website (in German) they state that they do release information to members of the public at a fee (they love that here).
A little bit of link-clicking later you will find that you have to pay 40 euros. I am not sure whether they can release the name of the owner.
The law (Luftverkehrsgesetz) governing the information in the register can be found here.

Airframes could be worth a try. Doesn't provide all the information you're looking for, but it's a beginning at least. I'm myself using it on work whenever I have to know an aircrafts type.

http://www.airframes.org

So, while this site is not officially sanctioned by any government, it is quite extensive. Accuracy and currency of data can vary, but there are other websites you can use to verify the source.
Finding single aircraft is the easy part. However getting an entire list of a specific country's registry, whilst I know is possible, may require you subscribe at a certain level to gain that functionality.
AeroTransport Data Bank (ATDB)
http://www.aerotransport.org

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Why does Turkish airlines still fly over Iraq, is it safe?
I realized that Turkish Airlines have a lot of flights over Iraq. For example, here is a flight from Istanbul to Maldives (TK 730).

Do you know if it is safe to fly there? Till today I thought that flight space over Iraq was closed for commercial flights.

Yes, this is last week flight, I saw it on flightradar. There are also a lot of others from Istanbul to Dubai, Riyad, etc

Well, only a small portion of western Iraq is currently under Daesh control, that flight flew over Kurdish and Iraqi government controlled areas, so I see how they might have declared those areas "safe"

Evidently, if there are "a lot of" Turkish Airlines flights over Iraq, and we are not hearing about any accidents, then I guess it is safe.

@J.Hougaard Well, we thought that about Ukraine until it all went wrong

Flying over conflict zones has been a common practice at all times. However after MH17 was destroyed over Ukraine some changes occurred.
From an overall standpoint, EASA, the safety agency for the European Union, has an active information bulletin dated March 31, 2017, which is the extension of one issued on December 23, 2015:

Iraq airspace, all altitudes.
  This CZIB applies to operators.
  This CZIB is valid until 30 September 2017
Due to the hazardous security situation, with the presence of
  terrorist organisations and ongoing high intensity military
  operations, there is a risk of both intentional targeting and
  misidentification of civil aircraft. The presence of a wide range of
  ground-to-ground and dedicated anti-aviation weaponry poses a HIGH
  risk to operations at all flight altitudes.
Recommendation(s):
Operators should take this information and any other relevant
  information into account in their own risk assessments, alongside any
  available guidance or directions from their national authority as
  appropriate.

It must be noted that Turkey while not a member of the EU, is still a partner of EASA and Eurocontrol.
On their side, the Iraqi Civil Aviation Authority states the sky over Iraq is safe.
Bottom line, operators have been informed of a high risk, and must evaluate their individual situation based on this warning and all other elements they can get on their own.
As mentioned by EASA, operators must comply with national directives if any.

For French operators, the situation is clear. DGAC, the national aviation agency has purely and simply forbidden Iraqi airspace (FIR ORBB) since July 2014, in an AIC valid for several conflict zones.
A similar order for Turkey would be issued by DGCA, but I was
not able to find such document.

In addition of Air France, many airlines have stopped flying in Iraqi airspace: press articles mention e.g. KLM and Qantas, Lufthansa and Royal Jordanian, and many other similar information can be found online.
The situation of Turkey is a little bit complex. Their have a southern border with Iraq and Syria, and the Sinai peninsula is also a conflict zone.

This may push operators to take more risks than others with southward routes.
About conflict zones, see also: Apart Kenya, what are the current dangerous / unsafe airspaces?

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How should I decide whether to pursue an EASA ATPL or an FAA ATPL?
What are the advantages/ disadvantages of FAA vs EASA ATPL's, and does this translate into the real world with airlines preferring one training standard over the other?
As a trainee pilot, it's pretty confusing looking at the array of courses offered around the world, with huge number of flight schools all advertising ATPL courses under different jurisdictions.
Any insight, experience would be very helpful.

Where do you want to work is an important question.

Do you have a car license? Which car license did you apply for? The FAA license will work in USA, the EASA license will work in Europe.

Where you have the ability to legally work is a critical component to consider too.

Both the EASA and FAA ATPL programs are good, I know people who have both and their view is that neither has an intrinsic benefit over the other. The EASA one has more theory in it which most of the ATPLs I know think is worthless, however if you know how to study it's really not that bad.
The question of which is best for you is who you want to fly for and where their airplanes are registered, as it's the country of registration which defines which ATPL you need, not the location the airplane is in. An FAA ATPL will let you fly US registered (N Register) airplanes anywhere in the world, and an EASA one will let you fly an EASA member's airplane, so most of Europe anywhere in the world. If you want to fly for a US airline you need an FAA one, if you want to fly for a european airline you need an EASA one.

@Ggd's answer is already good, but keep in mind that it's much easier to validate a EASA license to fly N registered airplanes than the other way around.
Validating a FAA license in Europe is a bureaucratic act and needs huge effort to do.
Also remember that, even as you can use your FAA license in Europe if it's validated, the rules are pretty different to those in USA so if you already know that you will fly in europe most of your time I would suggest a EASA license.


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Do modern jet engines monitor and record exposure to particles of dust or sand?
The BBC News article The Jet Engines with Digital Twins describes engineers monitoring measured versus modeled behavior of jet engines, and deviations between them signaling possible need of maintenance.
The following mention of sand:

One of the most useful parts of the digital twin is that it measures a huge number of factors that the engine faces throughout its lifetime -- some flights have more people on them then others (that will put more strain on the engine), some cities (like Abu Dhabi) have a lot of sand in their air, and some pilots push their engines harder than others. “With the twin...I can learn that the pilot is a cowboy and pushes the engine. The fuel burn we see will be different with different pilot. The digital twin remembers every one of those events. (emphasis added)

made me wonder if modern jet engines monitor and record exposure to particles of dust or sand. If so, roughly how do the airborne particle monitors work - do they monitor the size distribution, or just count number over a certain threshold of some kind?

I don't have sources to back it up but I believe the engines are checked regularly for damage, particularly the leading edge of the compressor & turbine blades, which gives a good idea of the amount of sand/dust ingested. I'm not aware of any sensors for such a thing. If a pilot is concerned about ingestion of large particles they could raise it with their maintenance teams.

@Notts90 Hopefully that pilot's concern would not have triggered by a sudden loss of power. I suppose on or near the ground air traffic control would be aware of a sudden increase in sand/dirt, and at high altitude a volcanic eruption would have been noticed and characterized fairly quickly. Anyway, a particle monitor system would quantify the exposure without a need for maintenance, but maybe there's no practical need for it in reality.

I've seen a turbine blade out of a eurofighter that had been to Afghanistan and was removed during overhaul because the sand had worn away too much of the leading edge. There was quite a lot of damage but apparently the engine still ran, just not as efficiently. So I think a sudden loss of power is unlikely unless you took in a lot at once.

@Notts90: There is likely some similarities between sand and volcanic ashes (though the melting point if different), the latter could shutdown an engine.

@Notts90 I spoke too soon! According to this answer a practical need in reality has been proposed and is indeed being considered.

As of now, there are no commercially available engine particle ingestion sensors, though a few are under development. One of the companies involved in development lays out the reasons for the absence of any such sensors:

As of today, no commercial off-the-shelf particulate sensor is in use because of the special design and difficult requirements for operation and performance in this harsh environment, according to Dr. Hai Lin of Hal Technology in Fontana, California.

There are multiple sensors under development like the Dual Optical Embedded Dust Sensor which is used to determine the particle ingestion by the AGT1500 engine in M1 Abrams, where dust and sand is a bigger problem, if anything.
All of these sensors work on a similar principle- they measure the scattering of light by the particles using a optical sensor.

The sensor detects light scattering from moving particles as optical pulsed signals whose amplitudes are proportional to size and whose number is proportional to concentration. This sensor technology can determine particle size, distribution, concentration and further be extended for composition.

In future most of these systems are expected to available commercially, as

While continuing to advance this technology for particulate composition determination, Hal Technology is pursuing a licensing agreement or partnership with sensor or engine manufacturers to have the sensor integrated into engine heath management systems of the future.

Thank you for the helpful links and background + current status information. Had the AGT1500 made it into a helicopter as a PLT27, it might have retained its particle sensor. According to the Wikipedia GE T700 article (selected over the PLT27), it featured "...an inlet particle separator designed to spin out dirt, sand, and dust."


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What is the material composition of aircraft compressor blades?
I read up that aircraft compressor blades are generally made out of titanium. I was curious as to why is titanium the choice of material used for aircraft compressor blades? What alloys are involved and are there alternative materials used in the designing of such blades?

Likely duplicate: What material is used to make the hot sections of jet engines?

@mins the compressor is the cold section

@Federico: Yes, but in spite of the title, the answers cover all sections of the engine, including compressor.

Titanium alloys are used as compressor blade materials in modern engines but I can't provide any official sources.

Simple answer is, Titanium is light and strong and is good enough for the temperature range of the compressor. JT3D, RB211, JT9D, Olympus, Spey, CF6 all use Titanium disk or blades. According to "The Development of Gas Turbine Materials, G.W. Meetham" page 73

But for high speed planes, the super sonic intake flow can get really hot when it reaches the engine, so the whole cold section could be 200K hotter than a low speed engine. This has been a difficulty because engines regardless of their use case, usually share the core (gas generator). E.g. D30 on MIG31 needs methanol spray in the intake to cool down the intake flow and protect the compressor, SR71's engine can not output full thrust because the hot intake would compromise the compressor.

@user3528438 Have you considered compiling your comments into an answer to this question?

Have found a reference to "high grade steel" and to titanium. According to the (paper format) book, the parameter that dimensions the compressor is the perimeter speed (tip speed) of the blades $u_2$.

For steel: maximum $u_2$ = 450 m/s. With a total inlet entry temperature of 300 K, pressure ratio is 4.2
For titanium which has a higher strength/mass ratio: $u_2$ can be up to 500 m/s, leading to a pressure ratio of 6.

The book was written at the time of the B757/767 engines so there may be more modern materials around.
Also refer to this answer.

Update
A more recent reference is provided by this presentation for the Trent 1000 engine:

Fan and LP compressor blades are made from titanium alloy, containing small amounts of Fe, V, O, and Al.
IP compressor: 8 stages of titanium blades.
HP compressor: 6 stages of titanium blades.

The reason titanium is selected is that up to a certain temperature, titanium has the highest specific strength (strength/weight ratio), and in the compressor the temperatures stay below the transition temperature for nickel alloys:

What is inlet entry temperature? 300K is just above room temperature.

Inlet entry temperature is the ambient temperature plus $\Delta T$ from compression in the engine inlet cowl

Most famous one is Nimonic 263, which is 50% nickel and 20% Chrome and created with additions of Titanium and Aluminium. Used in Concord engines.
https://en.wikipedia.org/wiki/Nimonic

Thank you. But according to the site you provided it applies to turbine blades due to its ability to withstand high temperatures. Would you happen to know which material would apply to the compressor section?


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What is the best strategy for landing when a flock of birds is on the runway?
At a controlled airport, the tower advises large flocks of small birds at the approach end of the runway.
I'm in a small trainer aircraft like a Cherokee or Cessna 172. Should I request to land the other way (Rwy 9 vs 27), proceed as normal but just expect them to only be a distraction?  What are the best options here?

You can climb much faster than they can. So if you stay above them (by a good 50-100ft) than you can either land long (if it's safe to do so of course) or do a low-approach and it might scare them away. Just keep your eyes out because more may be on the way to join them. When it comes to birds the most dangerous place to be is underneath them because they dive when they get scared. When they are on the ground the risk is minimal (if not on the runway you need) keeping in mind they will probably start flying when you get close. Also birds off to the side may cross the runway at anytime.

I think the answer depends on what the birds are doing, but all options are basically "at the pilot's risk" - the tower is just advising you there are some birds hanging around the airport, but nobody can say how they're going to react to you trying to land there too.

If the flock is airborne and flying across the final (completely neglecting to coordinate their movements with tower, as birds are wont to do) you can request an extended downwind until they've moved on.
This has happened to me twice, and in my experience birds tend to be far less interested in airports than pilots. They're usually gone in a minute or two.
If they're sitting on the runway somewhere you can try a low pass over the field to scare them off, but my preference would be to ask the tower/unicom operator if they can chase them away for me with a ground vehicle (so I'm not near the flock in case they decide to disperse "upward" instead of "outward").
If they're just pecking around in the grass near the approach end of the runway looking for lunch Skip Miller & p1l0t have the right idea with a go-around or low approach/overflight to scare them off.
(This is part of why a low pass over an unattended uncontrolled field is often advisable -- you'll hopefully scare off any critters on the runway that you wouldn't want there when you actually land.)
If they're "airport birds" you might want to just land.
Some of the airports by me have resident flocks of birds that I'm convinced just sit there and rate people's landings - I swear I saw the same birds every day shooting touch-and-gos.
These are the kind of birds that aren't going to scatter when you do a low pass unless you hit them with your landing gear, so just be aware of where they are (in case they lose their minds and try to commit suicide by propeller) but they probably won't bother you.

I don't think I'd ask for a downwind landing because of birds (unless we're talking geese or hawks or something), but if it's a calm-wind day, the pattern is empty, and you're more comfortable with that idea go ahead and ask. The worst the tower can do is say "unable".

"Airport birds" haha yes my home base is bordered on one side by a river. So not only is it flat, but being at sea-level, surrounded by a huge dyke with trees on top, it's also near a city, and a trash plant on the North side there are numerous birds on and in the vicinity with winter being the only exception. We have a bald eagle, huge swarms of starlings (they nest too everywhere), crows, seagulls, and wild turkey. Those are the "airport birds" too. We also get visits from hawks, Canadian geese, and others.

The best option is to go around.  Maybe the noise of you passing overhead will scare them into the air and they will land elsewhere.

The biggest problem you face in a propeller aircraft when going through a flock of birds is instant IFR. You don't want to do that on landing.
I had the experience of a flock of Canadian geese taking off as I was committed to takeoff while soloing as a student pilot. It was a spectacular mess, right out of a Friday the 13th Movie (or Indiana Jones).
I had to fly by instruments to climb to a safe altitude then maintain straight and level. Then I had to search the plane for some spare paper towels that I used to create a clear area in the windscreen so that I could see to land (reaching out the side window).
Thus, it is best to stay away from the birds. If they take off, you can abort the landing and climb. If the runway is long enough, you can land long, past the birds.

spare paper towels that I used to create a clear area in the windscreen so that I could see to land - grim


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What is this "hole" in the A350 fuselage?
As usual, I was looking at pictures of planes flying around me. And then I noticed a weird hole in this Singapore Airlines A350 fuselage:

Source
Registration number of this plane is 9V-SMD. According to the angle we look at the plane, the hole is more or less visible:

Source
What is this hole, and what is its purpose?

Different plane, same thing though: What exactly is this opening in the belly of a Boeing 737?


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Is there an official record of the pilot with the most logged flight time?
We often hear about pilots with 10 or even 20 thousands hours. So I was wondering how much higher this number can go. Do we know which pilot has the most logged time? Is there any official number?

Most airline pilots I know have about 25,000 hours when they retire.  I have been flying for 43 years and currently have just over 23,000 hours.

From Wikipedia, the free encyclopedia  John Edward "Ed" Long, Jr
John Edward "Ed" Long, Jr. (1915–July 18, 1999) was an American pilot who is in the Guinness Book of Records for the most flight time by a pilot: over 65,000 hours (more than seven years and four months) at the time of his death.
He began in 1933 at the age of 17, when he took his first and only flying lesson. In September 1989, he broke the previous record, 52,929 hours, set by Max Conrad in 1974. According to his brother, Ed Long's job involved checking power lines, so "most of that was under 200 feet, in a Piper Cub".He died in 1999 at the age of 83.*

This brings up the fact that it is really hard to compare what "hour" means. Some pilots, like this one, spend every minute with their hand on the stick, while a commercial pilot on an overseas flight even goes to sleep  (and, on the other hand, probably spends more time preparing the flight).

That is a good point. Some pilots, airlines, and jurisdictions, may not log flight time while on a break, while others do.

Aerial Patrol sounds about right for setting this record. I flew with a guy that has flown aerial patrol part time for the last 25 years and has averaged over 1000 hrs per year.

+1. Adding some dose of irony... In addition of being in the same book than Most-Socks-Put-On-A-Foot-in-One-Minute (Silvio Sabba from Italy) and The-Fastest-Ice-Cream-Scooper (Mitch Cohen, from Flushing, Queens), J. Long is also honored by the Smithsonian museum of Washington which emphasizes that "most of those hours were hand-flown, single-pilot, stick-and-rudder hours flown at about 100 feet above the ground." Not exactly the same than flying a 2017 aircraft.

The question is looking for an "official" record.
The answer is NO.  Nobody tracks this for all pilots.

Do you not accept the Guinness World Records as "Official"?   They list John Edward "Ed" Long, Jr. as the record holder with 65,000 hours at the time of his death.

Mike, no record should be considered 'official' unless it is independently verifiable.  Since pilots are not required to report their flight times to anyone, we are left with accepting what the pilot says it is.  It may sell some books for Guinness... and it may be very entertaining reading...  but since there is no verifiable source data, I don't consider the result 'official'.  But maybe that is just me...

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What happens to the cabin pressure if all the packs (ACMs) stop working?
I have gone through the cabin pressurization article in Wikipedia. It says there will be some number of Air Cycle Machines (ACMs, or packs) to supply pressurized air into the cabin.
What happens if all the ACM machines stop working? Does the flight have enough oxygen on board supply all passengers until they can safely land the flight somewhere?

The point of the oxygen masks is not to provide oxygen to passengers for the remainder of the flight, it is only to supply oxygen to the passengers until the aircraft can descend to a breathable altitude, usually only about 15 minutes of O2 per passenger is provided.

Even if all of the packs fail at once, the pressure will drop slowly. "Outflow" valves in the fuselage will close.  No rapid descent is required.  The crew will simply fly to an altitude which maintains cabin pressure.

Hi @TylerDurden, I have seen that sherpas carrying some oxygen cylinders with them (atleast for some emerygency). Pls Correct me if I am wrong.

@NANDA Those are just the ninny sherpas. Real sherpas hike to the top carrying Swiss "mountaineers" on their backs.

If all ACM failed, an alarm would sound in the cockpit. And two messages would appear on the ECAM/EICAS¹. One indicating that the ACMs failed and the other indicating that cabin altitude² is rising above desired level.
Pilots would execute corresponding checklist, and the one for rising cabin altitude calls for rapid descent to 10,000 ft. That is the altitude where humans can breathe safely.
If the problem is a hole in the fuselage, the oxygen masks would drop to allow breathing until the aircraft descends, and emergency descent (with full spoilers) would be done. But if it is just failed pressurization, the outflow valve would close and the pressure would only decrease slowly, so the aircraft would most likely descend before the masks would be needed and they would not release. Even normal rate of descent is usually sufficient in such case.
If you're flying above mountains, there are additional rules where to fly to avoid any terrain rising above 10,000 ft, which will be part of the flight plan. For example flying along the mountains of Peru, you'd need to deviate toward the ocean during such descent.
Now at lower altitude, the aircraft flies slower while burning more fuel, so it probably won't be able to make it to the original destination any more. However when planning flight and calculating the required fuel this is taken into account, so the aircraft has enough fuel that it can make it to one of suitably selected diversion airfields if it needs to descend to 10,000 ft at any point along the route.

¹ Modern aircraft have a display on which information about any problems and abnormal conditions is printed, which is called ECAM by Airbus and EICAS by Boeing. Older aircraft had large panels of warning lights.
² Pressure in the cabin is usually given as altitude at which that pressure occurs in standard atmosphere. Normal cabin altitude is 7,000–8,000 ft, 10,000 ft is maximum safe altitude (with generous safety margin). The oxygen masks fall out if the cabin altitude exceeds 14,000 ft.

Since the ACM's in most aircraft run off bleed air I assume the pilots can shut off the bleed? If they couldn't for some reason, how hot is the bleed air? I'm imagining something blocking the fresh air inlets. Would the cabin get uncomfortably hot or dangerously hot?

"Normal cabin altitude is 7,000–8,000 ft, 10,000 ft is maximum safe altitude. The oxygen masks fall out if the cabin altitude exceeds 14,000 ft." - could the pilot theoretically kill all the passengers by circling at 12,000 ft of cabin pressure?

@JanDvorak, unpressurized aircraft are required to have oxygen for the crew above 12,500 ft if over 30 minutes, above 14,000 ft always and above 15,000 ft for everybody (see https://aviation.stackexchange.com/q/25746/) and immediate danger is only above 16,000 – 17,000 ft. The 10,000 target is there to provide generous safety margin and to avoid risk of sensitive persons getting altitude sickness.

@TomMcW: If the ACMs have failed then the pilots open an emergency ram air inlet (usually located on the underside of the belly fairing), which uses the plane's speed through the air to force outside (fresh) air into the cabin, bypassing the ACMs. Depending on the outside temperature it might get a bit warm or cold in the cabin (controlling the cabin temperature usually requires the ACMs and bleed air from the engines), but the passengers will not suffocate, and this gives the pilots enough time to land at the nearest airport.


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Why does automatic trim exist only in the pitch axis?
Why does automatic trim exist only in the pitch axis? For example, a flight computer can hold the current pitch automatically when engine thrust is applied.
Why automatic trim not lies in roll and yaw axis?

I'm a little confused, "automatic trim" for the roll/yaw axis is usually referred to as the autopilot or wing leveler. What are you basing the assumption that aircraft don't have automatic control over those axis on?

@RonBeyer on some aircraft, trim is always computer assisted and does not require autopilot engagement.

In Airbus (A320+), the computer holds flight path angle (not pitch; the plane will pitch up to maintain flight path angle if you retard the power levers) and roll. And I am not aware of any other widespread civilian aircraft with auto-trim.

The pitch requires trim because pitch is related to airspeed and engine power. As both of these change quite frequently (except for the cruising phase of a flight), it is convenient to adjust the aircraft's "natural pitch" during climbs or descends, or after changing flaps.
Most aircraft* will not exhibit a tendency to roll / yaw to one side in flight, hence, it is already stable in these axis and trim is not required.
Automatic yaw trim does exist on some multi-engine aircraft. When one engine quits, the flight computer will automatically adjust rudder to compensate the undesirable yaw due to asymmetrical thrust. On an aircraft without such system, the pilot must manually use the rudder pedals, and the amount of rudder has to be adjusted as engine thrust or airspeed changes.
* Small single-engine airplanes do exhibit roll and yaw moments due to the torque and p-factor of the propeller, but these aircrafts are marketed for affordable price and easy handling, so auto-trim is not implemented.


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How much does a new hangar at a major airport save an airline?
I'm doing research on a case involving United Airlines trying to build a new hangar at Newark International Airport, and being extorted by the chairman of the Port Authority. He demanded an unprofitable, bi-weekly custom flight route in exchange for his approval.
As a small but important detail of my research, I'm trying to determine how much an airline can save by building a new hangar at a major airport.
I believe the primary benefit of a new hangar is the ability to do repairs and maintenance locally, instead of having to fly the plane elsewhere to do it. Is this true?
What expenses, roughly speaking, does a new hangar of this sort save a major airline each year (or longer period)? Sources would be appreciated.  So far I've only found information on the benefits to private plane owners in using or renting hangars.

This would be highly specific to a certain airline's operations, and even limiting to the case of United at Newark the relevant financial information is probably not public.

Does it save them anything? Seems like a cost center to me, just one that's necessary to operations.

@hobbs Why not? Just ferrying an airplane to another station is going to cost direct costs easily $5k-10k/flight hour, and costs of more downtime. Plus, what happens when a failure puts an plane in an unairworthy state? There's some balance involved.

@user71659, the airline is not going to ferry anything anywhere, they are simply going to organise their flights so the plane ends up in suitable place when it needs scheduled maintenance. So building a hangar at new airport does not really save money. What it does is allow better service at that airport.

Why on Earth would you call it “extortion”? It sounds like a normal business negotiation. The airport sets its price for selling or renting the space for a hangar and the airline can take it or let it be. And it even makes sense to demand opening some new connections, because opening new connections is what airline needs hangar for—current operation is doing ok with existing maintenance and new hangar is unlikely to help there. So it's not extortion.

What it all boils down is whether the conditions he set make sense for community (which would be all right), or just him personally (which would be bad, of course). Also it usually takes at least a year or two of operating a route to be able to tell whether it will be profitable in the long run or not, but if it makes sense for the community, it quite probably eventually would be.

@JanHudec https://www.nbcnewyork.com/news/local/David-Samson-to-Plead-Guilty-United-Airlines-Probe-Newark-Airport-Port-Authority-386723661.html


text:
Why aren't bird radars placed on the front of commercial aircraft?
Unfortunately, my skills and understanding of radar and the like are very limited, but if somebody could enlighten me why there is no such thing as a dual bird/weather radar to switch to?
I've understood there there have been some (successful) attempts with ground station. It would add weight, but i'd guess that perhaps would be offset for some time by a single ruined engine.

As an aside, putting the radar on would be easy. Actually avoiding the birds once you know where they are would still be really hard

Most big planes spend most of their time flying far above altitudes where birds are found.  So you're talking about hauling around this radar on every flight, but only using it for a few minutes on takeoff and landing.  Thats a very bad weight/utility tradeoff.

@abelenky Birds can be found pretty high. :-) But I do agree that a radar would be impractical beyond takeoff or landing. And those operations are better suited to airport systems.

I have "heard" that turning the weather radar on can cause birds to get out of your way, but I haven't seen any actual evidence to back it up.  I turn it on just in case it helps though since there is very little downside.  :)

@abelenky, as Shawn said some birds are indeed capable of flying very high however the largest risk of a bird strike is during takeoff and landing where birds are firstly more likely to be but also there is little to no chance of maneuvering -especially in an emergency situation. (and Lnafziger, dont let certain groups catch you saying/doing that)

@abelenky Your cost/benefit analysis is a little lacking. Most big planes spend most of their flying time with their landing gear retracted. So you're talking about hauling around these heavy wheels/undercarriage on every flight, but only using it for a few minutes on takeoff and landing. Just wanted to point out that reasoning by analogy isn't the best way to do things.

There really is no such thing as "bird radar".  Radar comes in a few frequency bands that have certain advantages depending on the distance, resolution and attenuation you need out of it.  The band isn't the only factor here as the band will dictate what size dish you need and your aircraft nosecone will but a distinct upper limit on that.  Weather radar is geared toward detecting rain drops and anything bigger than rain (hail, bugs, birds, etc) with reasonable distance and attenuation features (you want to penetrate into a storm).  Birds will show up on this radar, though depending on smoothing and processing algorithms you may need a flock of them before they get drawn on the radar, at which point they'll look like some rain.  A radar optimized to detect bird sized objects would probably use a wavelength that did not let you see things as small as rain drops and small hail.
Some other things to keep in mind are that the radar is not always on, and it should be off on the ground except during takeoff when needed.  The reason for this is that you do not want to be putting radar energy into people, and they tend to be in the way on the ground.  We only turned the radar on for takeoff when there was significant weather around the field and wanted to be prepared for requesting diversions.  Aircraft radar tilt can also be adjusted, so we wouldn't be looking at birds in front of us, but clouds above and in front of us.
Wit that said, our SOP on fields with known heavy bird activity was to takeoff with the radar on and pointed forward because someone had the idea that the birds didn't like being in the radar beam.  I have no sources to back that idea up, but it was written into our FOM.
Now assume you can takeoff and see a flock of birds on the radar.  Do you have enough time to do anything about it?  If so, what do you do, some maneuver away from them or do you turn into them in hops they take the evasive action?
Now back to the title question: Why aren't bird radars in the nose cone?

The weather radar can see birds, it would be redundant.
A bird radar (optimized for birds) would not be good at seeing weather, so where would the weather radar be placed?
In a real situation, what would a bird radar enable you to do with a positive detection with the time you have to react to it?  Probably not much.
Weather is a much more common threat than flocks of birds, so if you can only have 1 radar system, you want the weather radar.

The fact about attenuation makes sense. It would be very difficult to single out the birds from other stuff, and benefit in general would be marginal. the birds getting scared by weather radar sounds like something for mythbusters :) thanks for the answer

Supposedly the MiG-25 had the ultimate solution. Its radar was so powerful it'd kill any bird in its path, clearing itself a way through the flock :) But it also killed people, so it was decided to limit its use to above a certain altitude to prevent accidents.

Wow, that's quite the radar. Did it operate on 2.4 GHz and boil the bird or something? It seems like you'd have to be really close (or have a very, very focused beam) to concentrate that much power onto something the size of a bird, since received power is inversely proportional to the square of the distance from the transmitter. Unless, of course, you're using a laser. I'm guessing that the YAL-1 could take out birds pretty easily, though. :) That would be fun to watch, as long as you didn't accidentally hit the ground.

In a real situation, what would a bird radar enable you to do with a positive detection with the time you have to react to it? Maybe throttle down the engines for a few seconds to avoid sucking in the birds.

I don't know that a radar would provide enough of a warning anyway, if it could even see a single bird. And all it takes is one. The majority of birdstrikes happen near the ground, and the things I've seen have tended to be more to keep birds away from airplanes rather than trying to dodge them. They use cannons/noise-makers at many airfields and other means to keep birds away from low aircraft. And I've heard that the spiral painting on the compressor cones of many big jet engines, in addition to giving a visual indication that the engine is running, has a side benefit that, to a bird, it looks like the eyes of a predator and tends to scare the birds away from the engines. There have been mixed studies on the efficacy of that claim, but I don't know that any scientific research has been done to give a real yes or no. Boeing will claim that it doesn't matter. JAL claims that they had fewer birdstrikes on aircraft with painted spinners. I don't really know if it works or not.
In my experience, you aren't going to see-and-avoid most birds. You're simply moving too fast for either of you to get out of the way. I quit flying with only a few thousand hours, and I've hit several birds. And I don't know very many pilots who haven't either hit one or had a near miss. I've even been hit by a goose in my door when I while I was teaching in a 152, one of the few times that I can actually say the bird hit me and not the other way around.
The best way to keep from hitting birds is to try to avoid areas where birds are known to be flying, especially during migratory seasons. If you hear reports of a flock of birds at your airport, use a lot of caution. And report them if you see them. Try to minimize low flying. In most small craft, it's unavoidable. Your normal operating altitude is fairly low and right within the area that birds tend to fly. In bigger aircraft, try not to dally at lower altitudes, if you can help it. Climb to your altitude And if you're low to the ground (landing or taking off) don't put the aircraft in danger by trying to dodge birds. And every takeoff should include an idea of what you're going to do if you do hit birds.
From http://www.birdstrike.org/

"About 41% of reported strikes with civil aircraft in USA occur while
  the aircraft is on the ground during take-off or landing and about 75%
  of strikes occur at less than 500 feet above ground level (AGL).
  However, over 4,200 strikes involving civil aircraft at heights above
  4,500 feet AGL were reported from 1990-2013 in the USA and over 430 of
  these were at more than 10,500 feet.  The world height record for a
  bird strike is 37,000 feet (Griffon vulture off the coast of Africa)."

This first paragraph has some good points, but cites no references and, overall, the answer digresses from the point of the question which was specifically: why aren't there bird radars? Other methods of keeping birds away, while interesting, isn't really what was being asked.

Wow, 37000 ft bird strike. That is awesome. I mean, not the bird strike, but the fact that there are birds flying at 37000 ft.

@Bret Copeland I'm not sure what references you were looking for me to cite or how my answer digressed from the question. My answer was primarily based on my own experience, but I did provide a citation to birdstrike.org to back up my statement that most bird strikes happen near the ground. My answer demonstrated why a bird radar was impractical on an airplane and how there are much better resources that require little more than looking out the window and using the radio. The justification for a bird radar just isn't there. It wouldn't do you much good.

The weight of the bird radar would be trivial and it could easily fit in the nose cone; they aren't that big.  They don't have to look nearly as far out as weather radar,  All they have to do is indicate altitude and direction.  Aircraft can easily avoid collision by changing altitude.  They only have to miss the bird by being a few feet higher or lower, and birds oftentimes fly level at the same altitude, especially when migrating.  The computer could be programmed to avoid collision by adjusting the angle of ascent or descent.

Hi Andrew, welcome to Aviation.SE. Your post here does not really answer the question, it rather says it could be done, but doesn't answer the OP's question on why it is not done. Check out the Help Center on proper answers.


text:
I just flew into class B airspace without a clearance, what should I do now?
Hypothetical: Let's say (for whatever reason) a pilot became distracted or disorientated, and found themselves in class B airspace.
What steps should they take immediately and after landing to be safe and avoid any backlash from authorities?

Now that its been quite sometime, am eager to know your experience, how you handled it. It might point to some important areas where others can learn from.

One thing you should not do: go on Stack Exchange while flying to ask for advice.

While still in the airspace, you should contact the controller if you can, since it may be important to safety.
After landing, you may get the dreaded phone number from a controller (probably tower or ground) which you're supposed to call and speak with someone from the FAA. You should not volunteer any information about the incident during this call without talking to a lawyer or AOPA legal services first.
Then, most importantly, fill out a report in the Aviation Safety Reporting System. Any information you submit will not be used against you in an enforcement action by the FAA, and in many cases, people who volunteer information through the system receive much softer treatment, and sometimes forgo enforcement all together.
The reason the FAA incentivizes this self-reporting is so they can better collect information about accidents which inevitably happen. If they know why you accidentally strayed into the bravo, maybe they can improve procedures or charts to help keep other pilots from making the same mistakes.

This answer is very much focussed at the US situation. But I think in general the procedure is very similar in many countries.

@DeltaLima you're absolutely right. If you, or someone else, have specific advice for international flights, I think an additional answer would be very welcome here.

@BretCopeland I'd love to add an answer if I could, but this is not really my field of expertise. Thank you for pointing out the use of the safety reporting systems,

I would add that when entering the ASRS report, do not specify the infraction in the title. Powers that be cannot fish through report content looking for actions to take against pilots, however my understanding is that if they believe a title contains enough detail, they can pursue action they would otherwise have been unaware of.

@mah I don't have any way of knowing if that has any truth in it, but it would seem to be contradicted by the ASRS CONFIDENTIALITY AND INCENTIVES TO REPORT.

@BretCopeland I too have no way to know for certain... just what I've heard, and when faced with what seems to be a zero-risk/zero-reward vs unknown-risk/zero-reward scenario, I take the zero risk option.

Note that there is no legal requirement to actually call the tower when they ask you to, and some recommend to just call your lawyer instead since anything that you tell them is recorded and may be used against you.

I've done it three times in 1200 hours of flying, I must admit.
First time, my plane was performing better than usual (conditions were just right) and I nicked the SFO airspace on climb-out. About 20 minutes later, they called me with a phone number to call when I landed. Spent the rest of the flight shitting bricks. When I got to my destination, I was chewed out by the controller for about fifteen minutes.
I was so rattled I didn't leave my home airport again until I'd gone up with an instructor for a refresher.
Second time was in Boston. I flew through an airspace extension that wasn't on the map. Spent a few minutes on the radio arguing with the controller about it. "It's not on my map". "Well, we have a letter of agreement with the local airports." "But it's not on my map, how was I supposed to know?". Nothing else ever came of it.
Lesson learned: the space between the top of a class D and the bottom of the overlying class B airspace might be considered part of the class B, so just avoid them unless you're under ATC control.
Third time, I was in contact with ATC, skirting outside the lowest layer of their class B. I turned left to avoid a cloud when I should have turned right, and got chewed out by the controller and told to make an immediate right.
Anyway, the bottom line is that if you didn't actually endanger anybody, and just nicked the corner of the airspace (which is what I did in all three cases), you'll probably just get away with a stern talking-to.
But blunder through the path of an incoming airliner, and you're probably looking at a 4-month suspension of your license.
Filing an ASRS report is probably a very good idea if it ever happens to you.

was there a NOTAM about the extension? did you ever find out how you were supposed to know about the extension? I would be curious to know which Class D under the BOS Class B you're talking about.

Now that you mention it, there could well have been a Notam that I didn't read. It was a very long time ago, and so I don't remember the details.

There's a note in the Boston TAC to contact approach when flying above 2500' over OWD, BED, and BVY. You don't have to get clearance, but they do want you to be in contact with them (I just do flight following over there).

Yeah, I'm a big fan of flight following. I wish there was a way to file a VFR flight plan in advance and then activate with ATC to get flight following.

Since this question doesn't specify whether the pilot is operating as VFR or IFR or if the pilot has had previous communications with ATC, my best recommendation is to directly exit the airspace and then contact ATC for clearance.
If you are on an IFR flight plan and following a previous clearance, I would continue on your last assigned clearance and contact ATC as soon as possible. I would not recommend landing inside Class-B airspace without clearance unless you have a communications failure. Even with a failure, it might be more practical/safe to land to the nearest destination outside of Class B.
Upon landing, fill out an ASRS report and, depending on the infraction, seek legal advice in some form like AOPA Pilot Protection Services.

If you are IFR and following a previous clearance, you are allowed to be in Class B airspace, so this would be a non-issue....

FAR 91.131(a)(1). The operator must receive an ATC clearance from the ATC facility having jurisdiction for that area before operating an aircraft in that area.  It depends on who issued your last clearance and to what extent you where cleared.

Yes, when you are IFR you are operating on an ATC clearance so no specific class B clearance is required.

Yeah, when you're flying IFR, under ATC control, airspaces basically don't exist for you.


text:
What are the effects of different seasons on flight training?
What effect will the time of year a student starts their flight training have on the difficulty of their training?

What particular problems will each season cause a student pilot?
What advantages does each season have?
Are there any studies showing how the time of year effects the duration or success rate of training?

Different latitudes will have different, or at least more extreme, effects so please note if an effect is only valid for a specific geographic area.

Pretty much the only thing that is going to change season to season is the weather. Depending on where you are, there may be more aircraft traffic around a particular time of year, but I can't answer that for sure.
Obviously, the further from the equator, the more extreme the seasons. If you are close to the equator, it probably doesn't matter what time of year you start, since the weather is pretty much the same year-round.
If you are not at the equator, you need to consider the pros and cons of each weather season. Additionally, at extremes from the equator (near the poles), summer and winter months have extremely long days and long nights, respectively, which can make night and day flying difficult.

In the winter, the aircraft has better performance (airplanes like colder weather). However:

You have to deal with ice and snow, which is especially dangerous to light airplanes, which are often not as prepared to deal with them. It is dangerous in some degree to all airplanes
You have to be careful of low temperatures when starting the airplane before you take off

In the summer, the aircraft has worse performance (they don't like hot weather).

Many small airplanes have poor airflow inside the cockpit, which means they can get rather hot (it is easier to run a heater inside a small cockpit than cool it)
You have to be more careful to avoid overheating the engine
Some places may form thunderstorms constantly

In the rainy season, which is often in the spring, you will have degraded performance. Even if it is not rainy, the air will likely be more humid, which decreases aircraft performance

Depending on your latitude, other weather conditions may be more prevelant at a certain time of year (hurricanes/cyclones, high winds, tornados, sudden storms). This paper discusses wind differences based on season and latitude. In particular, page 7 of the document shows that summer months had slightly stronger winds at southern latitudes (southern hemisphere), and winter months had significantly stronger winds at northern latitudes. Some geographical areas may have changes in prevailing wind directions, which could also affect training depending on routes and runways too.
TL;DR: It's mainly a weather concern, which can affect your comfort (temperature inside, possibly air turbulence), aircraft performance, as well as grounding weather. Additionally, the most obvious concern is also length of the day - summer months will have more daylight, a particular concern if you can only fly in VFR conditions/do not hold a night rating.

Training in some places (like Florida) can be more difficult during the summer because of the large thunderstorms that form every day...

Night training can be tricky in latitudes near the poles in summer, because the sun may not depress far enough  below the horizon to technically count as night.

As SSumner says, the weather is your factor for season difficulty in flight training, and in large part these weather differences can be related to geographical location.
In the North US in winter time you have ice threats, for flight training ice is a no-go situation (most trainers are not approved for ice nor do they have a/ice capabilities).  For days with no ice, there is often an infinite plane of stratocumulus clouds that you need an IFR clearance to get above.  This can still be a go situation for maneuvers but makes ground-based navigation impossible to practice.  You also will typically encounter very strong winds as the weather clears behind mid-latitude cyclones.
In the South during the winter you have sporadic freezing rain and icing events, but without the frequency of the North. Your big threat instead are the spring and summer thunderstorm season.  On many days, if you respect the AIMs idea of tstorm avoidance, you might as well stay on the ground, because it just doesn't exist.
In the Florida You will have thunderstorms like clockwork daily, all year long.  You can fly before and after these storms and the sea breeze that drives these storms makes the timing and position fairly reliable to be avoided.
In the desert southwest you are dealing with high density altitudes due to the elevation and the heat.  This can make climb performance problematic but this can be remedied with a capable airplane.  This region can also be problematic for instrument training because you'll never get to log any actual instrument time.
In Texas north though Nebraska you can sometimes get intense low-level jets bringing moisture north out of the gulf.  This can result in very strong winds at the surface and gusty conditions in lowest few thousand feet, and this can be problematic for training.
In the lee of mountains with strong winds aloft (e.g. New Mexico north along the front range of the Rockies) you can experience turbulence, mountain waves and rotor clouds.  These can predicted by the winds aloft and can be avoided but may present days when training is best not conducted.


text:
What does flight categories "C", "G", "T" etc. mean?
In a flight database that I'm working with on a project, there is a column of data called "flightCategory" with values "C", "G", "T", etc.  Any idea what those actually mean?  From what I understand, the database is from FAA.  But I'm not 100% sure.

What is the database and what other columns are there?   The context could help us.  What are the rest of the values?  Can you post the database?

Grasping at straws, I'm going to guess flight plan, and type of flight, 'G' being general aviation, 'C' I think used to be commercial, being replaced by 'S' for scheduled and 'N' for non-scheduled. But I'm not a flight plan connoisseur so I might be off the mark. Still leaves 'T' though.

I got the answer.  'C' is commercial, 'G' general aviation, 'T' taxi, etc.
Update:
http://www.faa.gov/air_traffic/publications/atpubs/fss/AppendixA.htm, under section "Type of flight", it lists

S  if scheduled air service
N  if nonscheduled air transport operation
G  if general aviation
M  if military
X  if other than any of the defined categories above.

But types C and T isn't there.

Without context it's hard to determine if this answer is useful (at least "useful to anyone who isn't you") -- can you tell us more about the database you're working with?

Initially I only know these abbreviated codes "C", "G", etc.  and nothing else.  Later I was told what they actually mean.  The relevant engineer dug the information up from comments in source code.

You're missing the point of context - What database is this out of?, Who created/maintains it? etc. - without that context this question and answer aren't useful to the general public.

That, honestly, I don't know. The only thing I've been let know is the data is from FAA.

and where would you have found it? how does it look like? how is it named?


text:
Where does the 90 seconds limit for plane evacuation come from?
New airplanes have to confirm that all passengers have to be evacuated within 90 seconds or they wouldn't be certified by aviation authorities. But where does the 90 seconds limit come from? Why isn't it 60 or 120 seconds? Is it based on a series of studies or is it just a nice round number that was chosen based on a rule of thumb?

The origin is from tests done in the late Fifties. Flashover occurred after a bit more than 90 seconds.

@PeterKämpf that sounds interesting. Could you add an answer?

Related: the graphic in this answer

Sorry, my memory placed the test in the late Fifties when they were actually performed in the early Sixties. See full answer below.

The question has attracted my attention because of the successful evacuation of JAL 516.

@Peter-ReinstateMonica as per Wiki it took 18 minutes to finish evacuation in that case, way beyond the standard.

@JonathanReez Yes, I stumbled about that one, too. It includes the sliding phase, but still, the actual evacuation must have taken longer than 10 minutes, if that number is correct. Still, even if: The hull withstood the fire long enough to permit that evacuation -- surely the result of an according design.

Early research on post-crash fires focused on their causes and development, but soon it became obvious that the best way of reducing fatalities from post-crash fires lay in speedy evacuation of crew and passengers. Quoted from  DOT/FAA/AR-95/84:

In April of 1964, the FAA crash-tested a Douglas DC7 transport aircraft to examine the postcrash causes of fatalities. This test was followed by a crash test of a Lockheed L1649 transport in September.

Here is the link to a 1964 FAA documentary movie on the DC-7 test, and this links to a short excerpt from a documentation on the L-1649 test. Actually, the test was performed by the Flight Safety Foundation in Phoenix, AZ, under contract from the FAA.
DOT/FAA/AR-95/84 continues:

During the same period the FAA initiated research to prevent two types of postcrash fires. […] The second is a "flash over" condition which occurs when the cabin interior reaches a high enough temperature that the entire inside instantly ignites. […] Much of the FAA's Research and Development (R&D) has been dedicated to gaining the time by preventing or inhibiting the fires and by faster evacuation procedures.

It was observed that a structurally sound cabin remained habitable with the fuel fire raging outside until at about two minutes into the fire, when heat in the cabin became so intense that a flashover condition developed. This flashover was seen as the limit to the time available for evacuation.
Amendment 121-2, issued on March 3, 1965 and titled "Regulations, Procedures, and Equipment for Passenger Emergency Evacuation; Flight Attendants; and Assignment of Emergency Evacuation Functions for Crew Members" mandated rules requiring that passenger evacuation had to be accomplished in two minutes, using only half the available exits. It also marked the start to mandatory pre-flight briefings on emergency procedures. It in turn was based on the 1963 FAA notice of proposed rulemaking NPRM 63-42.
NPRM 63-42, which preceded the aforementioned tests by one year, proposed an evacuation limit of two minutes and the use of ropes to speed up evacuation:

In determining an equitable time factor applicable to all accidents requiring emergency evacuation of the airplane, the Agency recognizes there are many variables which can have a direct effect on the success of the evacuation. However, in order to assist the air carriers in having a means by which they can establish and subsequently measure their ability to execute the evacuation procedures, the Agency proposes that a time factor of 2 minutes to evacuate the airplane be specified

The evaluation of the 1964 tests and later research led the FAA to shorten the evacuation time to 90 seconds. Also, it was found out that ropes actually delay evacuation and slides were mandated to replace them. DOT/FAA/AR-95/84 again:

In 1967, the FAA initiated research to develop safety fuels to reduce postcrash fires and other requirements that addressed improved interior lighting, evacuation slides deployable in 10 seconds or less, improved exit distribution and excess exits, self-extinguishing interior materials, and protection of fuel and electric lines. These changes were coupled with a reduction of the required time for evacuation from 2 minutes to 90 seconds.

And that has stayed the same ever since.

90 seconds comes from a safety aspect, specifically a fire safety aspect
Aviation Safety Study SA9501 Footnote 3 says:

Internationally, industry accepts 90 seconds as a reasonable estimate of the survivable time in an evacuation where fire is present.

Which means that it is predicted than a burning aircraft is not sustainable for life beyond that period, but interestingly this is not the condition that the aircraft is certified under.
FAA Chapter 77 Section 1.7 says:

A full-scale emergency evacuation demonstration simulates an aborted takeoff. The operator must show that the aircraft, emergency equipment, and emergency procedures allow the evacuation of the aircraft at full seating capacity, including crewmembers, in 90 seconds or less.

So the 90 second rule comes from a fire safety aspect - as humans are not expected to survive in those conditions for longer than that; but the aircraft is only tested on whether it can evacuate on an aborted takeoff (not specifying that a simulated fire must be the cause of this aborted takeoff).
Sources: https://www.skybrary.aero/bookshelf/books/2699.pdf
http://fsims.faa.gov/WDocs/8300.10%20Airworthiness%20Insp%20Handbk/Volume%202/2_077_00.htm

Sure, but why exactly is it 90 seconds? Who came up with this number and how?

@JonathanReezSupportsMonica the quote dose say "a reasonable estimate", so I presume it's an estimated number which, technically, answers your question. However, I fully support the fuller question of "how did they come up with this estimate".

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Have there been any cases where the lack of seatbelts while taxiing has resulted in a passenger injury?
Airline passengers are always asked to keep their seatbelts after landing, until the plane has reached the gate. But have there been any real world accidents where the lack of seatbelts has resulted in an injury or death of a passenger?
I’m only interested in cases related to planes used in commercial aviation, with a capacity of at least 50 passengers. I'm likewise only interested in injuries that were directly attributed to the lack of seatbelts after the fact.

Not an exact answer, so in comments:  The worse single loss of life was the collision at Tenerife.  https://en.wikipedia.org/wiki/Tenerife_airport_disaster  One of those aircraft was taxing and 61 people on board survived.  But, I don't know how many of those people were belted and how many of the others  were not.

If your seatbelt is fastened, you quite certainly aren't standing up, which is where the real danger comes in... a sudden stop from 20 or 30 knots of taxi speed could send a standing  passenger tumbling in some pretty awful ways.  The easiest way to  ensure against somebody standing up when they shouldn't, is to reinforce keeping the seatbelts fastened until stopping at the gate AND the "fasten seatbelt" sign is off -- because simply "stopping" can happen well before reaching the gate.  Valid question, nevertheless.

It seems unlikely. Taxi accidents are fairly rare and most people comply with seatbelt rules. It's also entirely possible that any such injury would not be attributed to lack of a seatbelt since the injured passenger is hardly likely to admit that they weren't wearing a seatbelt in the first place.
A search of the NTSB database for accident investigations under part 121 containing the word "seatbelt" yields 158 results. Most of them are related to turbulence, with most of the rest being in flight or on takeoff or landing.
The closest example I could find is Independence Air Flight 1784, NTSB accident #ATL05IA116, where the nose gear spontaneously collapsed while the airplane was parked at the gate. This was post-taxi and the seatbelt light was off, so this is plausibly a reason that seatbelts should remain fastened even after taxi until the airplane is fully secured.
This shouldn't be taken as evidence that seatbelts have never prevented injuries during taxi, of course. There have been plenty of times planes have been forced to stop suddenly or have hit each other during taxi, and each of these is an incident that could have become such an example if people hadn't been wearing their seatbelts.

A few years ago there was a video of a regional jet taxiing in front of an A380 which turned off the main taxiway but then had to stop.  The jumbo kept going, the pilot unaware that his wingtip would impact the rj's tail. The video looked like a toy airplane kicked across the floor, the way the rj reacted to the hit.  Seatbelts for all probably prevented a LOT of nasty injuries in that case!

@RalphJ Jeez, that's nuts! And a perfect example.


text:
Can jet airplanes operate on 'rough' airfields?
A question I've always wondered is: can jet aircraft - not turboprop in this case - operate from 'rough' airstrips. My definition of 'rough' is grass or maybe a smooth mud airfield. I know that there's the dry lake at Edwards AFB that is used by aircraft for testing.

Are aircraft such as the Boeing/ Airbus passengers series - 737, 777, 747 etc - designed for or have the capability to regularly handle these airfields?
Are there currently any operators using these kinds of fields?

I can understand that tarmac/ concrete makes for a much smoother and more pleasant surface along with the reduce risk of FOD, hidden cracks that wheels could get caught in etc.
Any other examples such as executive jets, fighter aircraft that regularly handle rough fields are more than welcome.

See Antonov stuck in mud

Or TACA Flight 110:  not the landing, the take-off...

@User58220 Good example, I didn't think of that. I suppose you're referring to the take off after replacing the blown engine right?

@RedGrittyBrick The Canadian TSB report says that was after an excursion from an asphalt runway (03/21 at Gander), so it's not an example of operation from a rough strip.

The main problem with most jet airliners is that the engines are placed close to the ground. If the field is not paved, this increases the risk of damage to the engines from ingesting FOD (foreign object debris).
There are, however, jets that can operate from un-paved fields.
The 737-200 has an option called a "gravel kit." Despite having engines close to the ground, this kit helps to prevent the engine from sucking in FOD, which allows the aircraft to operate off of gravel runways. It also includes an addition to the nose gear to help prevent gravel from getting kicked up. For this reason, airlines like Canadian North still operate multiple 737-200 aircraft in order to provide service to airports with gravel runways.
High-wing aircraft are better suited for this type of mission (see What are the pros and cons of high-wing compared to low-wing design?). The C-17 is a military airlift designed to operate from remote airfields. Its wings are placed above the fuselage rather than below, which allows the engines to be mounted further above the ground. The IL-76 is a Russian aircraft of a similar configuration that also operates out of unpaved fields.
Aircraft with engines mounted on the rear fuselage also protect the engines from ingesting FOD. The new Pilatus PC-24 jet advertises the ability to operate from unpaved strips. Older aircraft like the 727 or the Tu-154 have this engine placement as well, and both can operate from unpaved fields.

737-200 landing on a gravel runway. The gravel kit is visible as the small extension just below the front of the engine, and the addition to the rear of the nose gear.

Most aircrafts designed in former Soviet Union are also capable of operating from unpaved runways because they needed to service remote locations in Siberia that didn't and don't have paved runways.

Better to use a T-tail aircraft like a DC-9, B727, MD83, F-100, etc since their wings screen FOD debris.

@user2357 -- the Mad Dogs (MD90, I think) are the only T-tail I know of with a gravelkit option; Cobham Aviation has also developed a setup for the BAe146/RJ85, taking advantage of its high wing configuration.

The mechanical strength of an airfield can be measured in CBR, meaning California Bearing Ratio. Another way is Westergaard's theory, which uses a more analytical approach. ICAO has used both as the basis of ACN, the aircraft classification number which can be used to express the effect of a particular aircraft on the runway. The pavement in turn is classified by PCN, the pavement classification number. Here is a lengthy Boeing document on the calculation of PCN. If you know both values, you can determine how damaging the operation of a particular aircraft will be on a particular airfield.
Generally, civil aviation wants to use clean, hard surfaces. Tire pressure is a good indicator how high the tire load is, and how soft the surface may be. Commercial jets have 100 - 220 psi (7 - 15 bar) tire pressure and need a hard surface (concrete or asphalt; gravel operation is really unusual and needs lower tire pressure). Carrier-based aircraft have up to 350 psi (24 bar) tire pressure when operated from the steel deck of a carrier. On the other extreme, some military transports are still jet aircraft but need to land on unprepared surfaces, so their tire pressures go down below 100 psi (the C-17 uses 144 psi normally, but this can be lowered to 120 psi). 125 psi (8.6 bar) is also the regular inflation pressure of the C-130 E landing gear tires, designed for operation from unprepared surfaces. See here for a study (PDF!) on the pressure exerted on the ground by a range of aircraft tires.
At the low end are the tires of gliders which routinely operate from grass strips and are fine for landing in the next field. Their inflation pressure can be as low as 50 psi (3.5 bar).
As Jan mentioned, many Russian aircraft have traditionally been designed for operation from unpaved airstrips. Even fighters like the MiG-25 or the MiG-29 have (relatively) low-pressure tires (10 bar / 150 psi) which allows their operation from unprepared airstrips. The MiG-29 can even close its air intakes to prevent foreign object damage, sucking air through louvres on the upper wing. Here is a picture of the intake of a Polish MiG-29 in take-off configuration, taken from this website:

Right air intake of a MiG-29 with main doors closed and louvres on the wing root opened.
With the MiG-25, you could take off from a grass strip, climb to FL800 and fly at Mach 3, and land on that grass strip again. Fascinating!

A MiG-29 can hit 80,000 feet?

@ptgflyer: The picture is of a MiG-29, but the text mentions the aircraft which can zoom up to FL 800: The MiG-25. It's not a caption, maybe I should add more space.

Yeah, MiG-25 makes a lot more sense.

Isn't that the right air intake?

@jbg: Yes it is! Thanks for spotting this!

I can't vouch for other airliners, but the 727 was used on dirt airfields in Africa quite often.
Video of 727 taking off from a dirt strip

and then there's the VFW-614 which was designed with overwing engines specifically for rough field operations in developing countries.
It sadly failed to sell, and the aircraft is now largely forgotten.

The Honda HA-420 HondaJet  has over wing mounted twin turbofans. The location was claimed to be selected for noise and efficiency reasons. While the aircraft is unlikely to have been intended for unprepared runways, the configuration appears well suited for this.

@Pekka nice little aircraft. Seen sketches of the idea years ago when it had a different engine configuration. It doesn't look very well suited to unprepared fields with that very low landing gear, but if the belly is suitably reinforced against impact by stones, branches, etc. it might work (the Pilatus PC-9 is as low or lower and designed for such work).

There are several problems:
1) Whether the landing strip can handle the weight of the aircraft and landing impact(s),
2) Whether the aircraft landing gear itself survives repeated abuse and
3) How much debris does get sucked into the engines.
With regard to fighter planes, I know for a fact that MIG-29 fighters have several slits on top of their intake (and top of their wing, resembling a shark's gills) and can actually close the main intake when landing or taking off; This way you get much less dust operating from less-than-ideally maintained runways.
With regards to landing gear, there are special designs for that; Swedish SAAB  fighters have had longer landing gear to be able to land on the roads, refuel and rearm and fly again against the Soviets, or you could do the C-130 (I know, a turboprob, bear with me) and have multiple wheels to distribute the landing load so that the plane does not sink into the runway.

none of which problems are specific to jets...

The RNZAF have in recent years flown 757's to and from a permanent ice runway in Antartica . The runway is 10,000 feet long and has lights etc but I thought it might qualify as a "rough runway". There are lot of better pictures than the one I've linked to if you use an image search.
They had a difficult flight in October 2013 which has put into question the future of these operations. The difficultly was not related to the nature of the runway but to the lack of landing alternatives in the event the weather goes the wrong way.

Much is made by Pilatus of the capability of its PC-24 to land on unpaved strips, but the BAe/Hawker 125 has been able to do this since the 1960s. It operates in Africa and Australia with a so-called 'rough field' kit, enabling to land on any unprepared surface. The high engines and the robust construction of the aircraft in general allow this.
If anyone is any any doubt about this, please check this video on YouTube, showing the aircraft being recovered from an illegal airfield in the Guatemalan jungle:

The downside of is that the 125 has become the aircraft of choice for drug smuggling, precisely because of this capability.


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Why is there a maximum landing weight limit?
I was told that if the aircraft's weight exceeds the maximum landing weight, it could cause a structural damage during landing, but why?
My understanding is that with a heavier aircraft, I need fly with a faster approach speed; but if the runway is long enough, I could land with more lateral energy with minimized vertical energy without causing any structure damage, is that right?

Maximum landing weight (MLW) limit exists primarily to cater for approach climb performance requirements (i.e. go-around requirement). As per 14CFR 25.1001: if the aircraft does not have a fuel dumping system, it must meet the all-engine-operating and one-engine-inoperative climb in the approach climb configuration at maximum takeoff weight. Otherwise, credit can be taken for maximum landing weight. So the design decision of whether incorporating a fuel jettisoning system or not comes down to this.
Landing gears are designed to land above MLW and even at maximum takeoff weight, with or without fuel dumping capability. 14CFR 25.473 requires that landing gear and supporting structures withstand a landing load at maximum takeoff weight at a contact speed of 6 ft/s. This is definitely reduced from the 10 ft/s design load for maximum landing weight, but is still larger than the typical landing speed of 2-4 ft/s. However, maintenance inspection would be required if landing above MLW occurs.
The need for fuel dumping has been thoroughly discussed in this question.

Would I be correct in my understanding that Maximum Landing Weight isn't the maximum weight at which the plane can land with minimal likelihood of harm to passengers and crew, but rather the weight with which a plane can be expected to land routinely without requiring extensive inspection for damage?  If so, it might be good to mention that.  On a long runway, a skilled pilot who minimizes landing impact may prevent the plane from being damaged, but operational procedures would require inspection even though that inspection might reveal that everything was in fact undamaged.

@supercat yep, aircraft are performing overweight landing in exceptional scenarios, e.g. when something seriously bad happens after takeoff.

@yo': Is there any factor beyond the possibility of damage requiring repair before an airplane could be reused that would limit an aircraft's landing weight to a value below the takeoff weight?  A plane's takeoff weight needs to be such that a plane can safely get airborne even if an engine fails just after an aborted takeoff becomes impossible.  Or is your point that the maximum takeoff weight on one runway might be less than the maximum landing weight at another?

@supercat medical emergency, passenger posing safety threats, etc. for instance.

@supercat It depends on each airline and each airframe. At my airline, if the landing is going to be overweight, the pilots note the vertical speed reading at touchdown.  Maintenance can look up on a chart whether or not an inspection will even be required.  So, depending on how soft the landing is, an overweight landing may have no repercussions at all.

At the instant of touchdown, any vertical kinetic energy the plane possesses must be absorbed by the landing gear, which thereafter must bear the entire weight of the plane. That kinetic energy depends on the sink rate and the plane's weight.
The sink rate ideally will be zero at wheels down in a perfect world, but the gear is designed to accomodate a certain range of sink rates and plane weights via compression of its spring struts and shocks. If either the sink rate or the plane's weight causes the maximum compression range to be exceeded, the suspension will run out of travel and bottom out hard and the resulting shock can damage the attachment points of the gear to the fuselage or the gear itself- or even cause the gear to collapse.
The maximum gross weight at takeoff (full tanks, maximum payload) is set by engine power and wing lift when configured for takeoff and the expectation is that by the time the plane has reached its destination it will have burned off enough fuel to be under the allowable gross weight at landing (which is set by the design of the landing gear as explained above). This means that the plane's designers can minimize the weight of the landing gear by not having to strengthen it enough to routinely accomodate a landing at maximum gross weight at takeoff.
And that, in turn, means that if a fully-loaded plane has to make a quick return to the line for any reason, it must dump fuel first.

...and that for even quicker return the pilot can decide to overstress the landing gear if breaking the aircraft is not important in the current emergency.

Well, braking power also can limit the max. landing weight, and then it depends on the conditions (rwy length, winds, whether, altitude, missing thrust reversers, ...).

One other thing to take in mind, is that brakes convert the kinetic energy of the plane to heat after touchdown. And you don't want your brakes to overheat

But if you report you are coming in ‘hot’ I’m sure they will have something to ‘cool’ you down waiting.

@vasin1987 unless you insist it's “not an important fire”...

They still cool you down. It’s their job. Let’s get out of here.


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Can a private pilot refuel his small private airplane internationally without passing through immigration or customs?
I've been a private pilot for many years, but I have never flown internationally in small private aircraft. I'm about to buy a tiny, high-tech, super-efficient 2-seat airplane that has extended range fuel tanks that make it possible to fly 8000km == 5000 miles without refueling (24+ hours).  The longest flights necessary to cross the south-pacific in hops is 4000km (about 16 hours of flight at 250kph).
Such long range makes it possible for this tiny aircraft to fly anywhere on earth.  Nonetheless, I'd have to be nuts not to stop more often than every 16 hours when that is possible: for a meal, potty stop, walk around and stretch body & legs... and eventually sleep!
This raises the asked question.  Can a pilot, with or without one passenger, stop in other nations JUST to refuel without needing to go through immigration (and get a visa for that nation)... just to land, pull up to the gasoline pumps, fill them up, get back into the airplane, then fly away?
Note that this is NOT a question about flying in a commercial airplane on a conventional scheduled airplane flight. I also presume the stop will not involve going inside any building or leaving the refueling area within the private aircraft portion of the airport. It will involve getting out of the airplane, because refueling of diverse small private airplanes is generally done by hand by pilots, not by the huge trucks that fuel jet airplanes. Typically small airplanes just pull up to a pump much like you'd see at any gasoline station, insert credit card, pump fuel into the wings, then fly away.
When I get the airplane, after a month or two of getting used to it in north america, I plan to flying down to Chile with 1, 2, 3, 4, 5 stops between (depending on these rules), then fly to dozens of islands in the south pacific (and land many places and islands that have airstrips, but no buildings (or at least no legal/government buildings).
In the later cases (south-pacific islands with tiny populations), I suspect nobody bothers or cares, because they assume all air traffic is strictly local. But flying down through (or 20 miles offshore) of Mexico through central America then Columbia, Ecuador, Peru and then finally into northern Chile (where I intend to spend a great deal of time) may be another matter entirely.

Range is 5000 miles? What airplane is that?

What's it use for fuel?

The airplane is a Pipistrel Virus SW with Rotax 912iS engine flown at "economy speed" (roughly 240kph).  At "cruise speed" (about 275kph), more like 4000 miles.  Of course you need to buy the optional 300 liter "extreme range tanks" too.  The engine runs on premium unleaded automobile gasoline (without ethanol).  Can accept ethanol, with a reduction of about 5% fuel economy.  It can also take standard aviation fuel (100LL), but best to avoid that for longest engine lifetime.

I'm not sure where you got 4000 miles from, at 100 liter tanks and maximum range is 767nm, with 300 liters it would be just over 2000nm. Keep in mind that's theoretical and you wouldn't in practice get that far as you'd need reserves.

Also, if you are landing at international airports for fuel avgas is what you're most likely to find, mogas is generally available at the more LSA friendly airfields, even then it can be hard to find.

@GdD : You must be looking at the wrong model or wrong engine.  Note that the other Rotax 912 engines are much less fuel efficient than the computerized fuel-injected Rotax 912iS (though both are 100HP).  Check the link forum, the Virus and Virus SW section for the Virus SW100 real life performance message and you'll find the actual real-life experiences of real owners.  Note also that pilots of pipistrel virus sw aircraft have flown the 4000km "longest hop" across the south-pacific several times.  That is as real world as you can get.

@GdD : Yes, will need to research what fuel is available at what locations before leaving.  To get the best gasoline may well be a challenge, which is yet another reason having a very long range is quite helpful (more landing options).  To have contacts in advance at expected landing sites is helpful too... hopefully they can bring outside fuel to the airport [for "their" airplane].

I’m not sure but maybe your POH may contain figure of range. Can you post it when you get it?

I have the PDF document for the airplane, but it only gives information for 75% power "cruise speed" and maximums like Vne (302kph).  All information for "economy speed" comes from existing owners.  I have a friend who has one of these and gets 70+ mpg at her best economy speed (usually about 230kph~240kph at about 13,500 feet altitude).  For 75% power "cruise speed" and 100 liter regular size fuel tanks the PDF says 1550km range @ 275kph.  Extrapolate that to 300 liter tanks == 4650km range with nose-wheel version (or 4790km range my tail-wheel version).  But economy speed is much better.

@vasin1987 : I have the PDF, but don't see how to post PDF files here.  But the best information is in that forum.pipistrelowners.com forum in the "Virus and Virus SW" section in a topic title something like "real world performance".

You can screen capture it and post relevant page here.

I just looked at the formatting page and don't see how to make an image visible here without putting it on a webpage somewhere and linking to the image in the webpage.  Maybe I'm missing something.  Here, try clicking on the following internet link and see if you can download the PDF, then look on page 5:    https://www.pipistrel-usa.com/wp-content/uploads/2018/03/Pipistrel-VirusSWiS-Information-Pack.pdf

I would check with AOPA for the countries you are interested in landing in, and some intermediates as well in case of weather, mechanical problems, etc.  I have looked at www.skyvector.com for a flight down to Guadalupe, it shows airports and fuel availability & prices, so that might be another resources to check as well.  Looking at skyvector, a flight from southern Texas, KTX, Midvalley Airport, to SCDA, Diega Aracena, is 3227nm. I think I'd pick a route that didn't have such long stretches over water myself.  That's a long time in a small plane.

From the Pipestrel website:  While the performance numbers of the Virus SW do not change, as the 912 iS maintains 100 HP, the fuel consumption decreases dramatically by as much as 21%, preliminary numbers show. This means that the Virus SW equipped with the new Rotax 912 iS cruises at 274 km/h (147 kts) in 15 litres per hour (3.95 gph). What an amazing achievement! It can be said that the double NASA Challenge Winner Virus SW finally got the perfect engine!
3227nm/147kts = 22 hours in the air.  I'm pretty sure I couldn't do that and still be a good pilot.

@CrossRoads : That www.skyvector.com website is sweet.  Thanks for pointing that out to me.  Yes, I agree that a route that avoids open ocean is a better idea.  Even better that application lets me choose routes that pass over interesting terrain (mountains, lakes, etc).  I don't mind traveling 25% further and pay 25% more for fuel to fly over more interesting places and avoid open ocean.

@CrossRoads : The 912iS engine was improved a little further since that article, and they don't discuss the better fuel economy at "economy speed" which is about 240kph to 250kph versus cruise speed of 275kph.

only done that route ever in X-Plane and MSFS. Best bet is probably flying to Miami, then follow the Bahamas in the direction of the Dominican Republic and St.Maarten and to Guadeloupe from there. Lots of islands to put down in in case of emergency or to get something to eat and gas.

While it depends on the country, expect to have mandatory immigration checks on first port of entry in most countries. I know this to be true for the US, Canada, Britain, Russia and the EU (Schengen area). Usually only emergency landings are exempt. But they may lead to investigations so not really an alternative to declare one in each and every country.
Usually it's best to hire a service provider to prepare all the paperwork for your trip. While their main business is to prepare commercial flights (non airliner), most also accept private pilots as clients, a few even welcome them. They will not just plan your journey regarding visas, landing and crossing permits (some countries do require preregistration for each and every flight into their territory), but also make sure to pick only airports with the right fuel available, or making it available. Selecting and booking hotels can be included as well.
Anyone I know, who ever tried to fly further than the next door country, sooner or later took such service as it makes a huge difference. Well, some had to make the experiance to get stuck for a week at some border first :))

first point of entry is the crux here. If you never enter the country you don't have a first point of entry. And staying on board or on the ramp while a crew refuels you may or may not count as entry depending on the country and facility. I've had fuel stops (albeit as a passenger) many times in various places, and rarely had to go through customs in the process. Can't see how it'd be different if you're a pilot and just waiting for someone to "fill her up".

Take the US as example. When entering, you're right away directed to the immigration apron. There's no fuel or whatsoever, and you're advised to stay inside your plane, even on a fricking hot day, until they done their way. The US is one of the countries even checking passengers between international flights - they always check. Canada is the same albeit a bit more relaxed. In the EU (and other countries) handling varies as sometimes most of the time you're allowed to leave the plane and wait in their office buildings or alike.

Can you list a few of those companies... or at least provide the generic name for those types of companies so I can perform a search?  You mentioned US, Canada, Britain, Russia, EU... but my flying will be US, Mexico, all the others in central America, then the western part of South America (Ecuador, Peru, Chile, western Bolivia and Argentina).  Hmmm, maybe overfly Columbia up at the north end before Ecuador.  Those companies should know, so try to tell me who they are and how to contact them.  Thanks for that tip.

Since I never flown in South America (nor Mexico) or Asia, I can't give reliable information here. General term would be flight services or flight support.
Not sure if names are wellcome here, but examples might be World Air Ops in the US, FlightWorx in the UK or FSINT in Germany. While the former are more geared toward large customers, seams FSI quite willing to support everyone - at least I made good experiance when planing a world tour (which never hapened). There are many more, jsut do a web search, maybe with 'overflight permissions' as additional keywords.

It depends on the laws in the country you are landing in, but for the most part there's no reason you can't refuel without going through customs/passport control, commercial pilots do this all the time. They don't get out of the airplane for the most part but there's no reason they couldn't. Generally there's no reason to go through passport control unless you want to enter the general terminal area or exit the airport altogether.

That makes sense.  Would the pilot generally inform approach or tower the landing is for refueling only?  Also, would it be necessary to only refuel at POE (port of entry) airports?  Probably, huh?

When we fly from the Lower48 to Alaska, transiting Canada, the customs language from both countries specifically state to "not get out of the aircraft" until a customs official is present. This is most tricky when arriving in AK, where the most common first stop is Tok, which has no customs facility on-site, but to where customs agents will drive to from the border crossing when the flight is pre-coordinated. They state: "it is important to arrive as close as possible to your ETA, because you cannot exit the aircraft until we arrive."

@Jimmy : What if one flew into international airspace in Washington state, then re-entered in Alaska without entering Canada airspace?  Does that make anything easier?  Probably not, huh?  Nonetheless, fantastic if smaller airports in other nations do what you describe in TOK (Tok Junction Airport).

@bootstrap that happens all the time. "International Airspace" isn't the problem, nor is even flying through a foreign airspace. Most countries are part of ICAO, which have overflight agreements. Departing Chicago, flying over Canada, and landing in Anchorage, AK requires no customs upon arrival. It's only once you've had your boots on someone else's soil that the next country in line wants to talk to you upon entry.

@Jimmy : Obviously it makes sense that airplanes can fly over nations they won't land in without creating immigration / customs issue.  If that wasn't true, a great deal of travel by flying would be utterly intolerable.  From a similarly logical viewpoint, it makes sense that the runways, taxiways, aircraft fueling areas [and restroom] would be legally considered equivalent to "being up in the air" (overflying).  This would make stops for refueling nearly as non-problematic as overflying (from the standpoint of immigration and customs issues).  Maybe that's more-or-less how it works.  Thanks.

@bootstrap Nonetheless you may still need to get an approval ahead of flight time to cross over any country. Nations like to keep their airspace tidy, so even if you're intention is just to pass over, you need permission. Something not given en route at all (some special cases exist, but they are rather rare). So plan to get all fly over permits well before take off.


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When is a flight plan required?
Do all flights need to have filed flight plans or only ones that fit certain criteria? What are the criteria?

In the US you need to file a flight plan for the following flights:

IFR flights
Defense VFR Flights, which are required to fly in the ADIZ that is off the coast of the US (an IFR flight plan may also be used).
Some TFR's only allow air traffic to fly within their boundaries if they are on an IFR or VFR flight plan.
A DC FRZ flight plan is required to fly VFR in the DC FRZ.
A DC SFRA flight plan is required to fly VFR in the DC SFRA.
When entering or leaving the U.S., an ICAO international flight plan (IFR or VFR).

Other than that, no flight plans are required, but are strongly recommended so that search and rescue gets called if you don't show up at your destination on time!